Subterranean deposits of coal contain substantial quantities of entrained methane gas. Production of this gas is desirable both when it can be produced in useful quantities as a natural resource as well as when it is present in areas where mining of the coal is planned or in progress. Substantial obstacles, however, have frustrated extensive development and use of methane gas deposits in coal seams.
The foremost problem in producing methane gas from coal seams is recovery efficiency of the gas from the coal. Recovery efficiency in coal varies widely, and in coals where the recovery efficiency is low, vertical well developments obtain only a small amount of gas from around the well. Further, some coal deposits are not amenable to pressure fracturing and other methods often used for increasing gas production from rock formations. As a result, once the gas easily drained from a vertical well bore in a coal seam is produced, further production in some coal is limited in volume. Additionally, coal seams are often associated with subterranean water, which must be drained from the coal seam in order to produce the methane.
Horizontal drilling patterns have been tried in order to extend the amount of coal seams exposed to a drill bore for gas extraction. Such horizontal drilling techniques, however, require the use of a radiused well bore which presents difficulties in removing the entrained water from the coal seam. In most instances, pumping water from a vertical bore is more efficient and less expensive than pumping water from a horizontal or radiused bore.
Systems based on horizontal bores that intersect the cavity in a vertical well bore combine the advantages of horizontal drainage patterns with the efficiency associated with pumping from a vertical well bore. Liners, often installed to enhance the structural integrity of the bores, are joined at the intersections between lined bores by junctions installed using various techniques.